[Build] Clarify `BuildPlan.Error.missingLinuxMain` (#1920)
[Build] Clarify `BuildPlan.Error.missingLinuxMain` (#1920)

* [Build] Clarify `BuildPlan.Error.missingLinuxMain`

Make it clear that LinuxMain.swift must be at the top level.
[SR-8646] BuildPlan.Error.missingLinuxMain description rephrasing

* [Build] Reclarify `BuildPlan.Error.missingLinuxMain`

Make it clear that LinuxMain.swift must be in the Tests directory, rather than just an arbitrary "top level".
[SR-8646] BuildPlan.Error.missingLinuxMain description rephrasing

abertelrud/swift-package-manager,apple/swift-package-manager,abertelrud/swift-package-manager,JGiola/swift-package-manager,abertelrud/swift-package-manager,JGiola/swift-package-manager,apple/swift-package-manager,JGiola/swift-package-manager,JGiola/swift-package-manager,apple/swift-package-manager,JGiola/swift-package-manager,abertelrud/swift-package-manager,apple/swift-package-manager,abertelrud/swift-package-manager,apple/swift-package-manager